1. Field of the Invention
The present invention relates to a semiconductor laser device which emits laser light having an arbitrarily variable wavelength. In particular, the present invention relates to a variable-wavelength semiconductor laser device which contains a tapered-stripe semiconductor laser amplifier as a light source, where wavelength selection is performed on laser light emitted from the tapered-stripe semiconductor laser amplifier, and then the laser light having the selected wavelength is returned to the tapered-stripe semiconductor laser amplifier to be amplified therein.
2. Description of the Related Art
Various attempts have been made to obtain a high-power laser beam having a single wavelength by utilizing a semiconductor device. As one of such attempts, a semiconductor laser device is disclosed in Electronics Letters, vol. 29, No. 14 (1993), 1254-1255.
As illustrated in FIG. 7, the above semiconductor laser device contains a tapered-stripe semiconductor laser amplifier 1 as a light source, and laser light emitted from the back end surface la of the semiconductor laser device 1 is collimated by the lens 2 to be incident on the reflection grating 3. Only the laser light 4 having a single wavelength is reflected by the reflection grating 3 to be returned to the tapered-stripe semiconductor laser amplifier 1. Therefore, the wavelength of the laser light 4F emitted from the front-end surface 1b is locked to the single wavelength. Thus, the variable-wavelength semiconductor laser device of FIG. 7 can output a laser beam of a high output power (not less than 1 W) and high quality (close to a diffraction limit).
In the above semiconductor laser device, the selected wavelength can be changed by rotating the reflection grating 3 in the directions as indicated by the arrows A in FIG. 7. Thus, the oscillation frequency can be changed.
In the semiconductor laser device in which the oscillation wavelength is selected by a reflection grating, the oscillation wavelength can be changed over a considerably wide range. However, there are drawbacks. That is, since the reflection grating is rotated by a mechanical driving means, downsizing of the device and precise tuning are difficult. In addition, the output is liable to become unstable, for example, due to misalignment of the constituents of the semiconductor laser device.
Japanese Unexamined Patent Publication No. 10(1998)-190105 proposes a semiconductor laser device in which a birefringent filter is provided as a wavelength selection means, instead of the above-mentioned reflection grating.
This provision is made in order to lower a threshold current for oscillation and increasing luminous efficiency. In addition, Applied Physics Letters, vol. 73, No. 5 (1998), 575-577 discloses a semiconductor laser device using a fiber grating as a wavelength selection means, instead of the above-mentioned reflection grating. In these semiconductor laser devices respectively using the birefringent filter and the fiber grating, the oscillation wavelength can be slightly changed by changing a driving current.
Since the semiconductor laser devices using a birefringent filter or fiber grating do not contain a mechanical driving means, the aforementioned drawbacks of the semiconductor laser device using the reflection grating do not exist in the semiconductor laser devices respectively using the birefringent filter and the fiber grating. However, the variable range of the oscillation wavelength by changing the driving current is very small, i.e., practically, the oscillation wavelength is not variable in the above semiconductor laser devices using the birefringent filter or the fiber grating.
The object of the present invention is to provide a variable-wavelength semiconductor laser device which contains a tapered-stripe semiconductor laser amplifier as a light source, emits laser light having a variable wavelength, and realizes downsizing, precise tuning of oscillation wavelength, and a stable output.
The above object is accomplished by the present invention, which provides a variable-wavelength semiconductor laser device containing a tapered-stripe semiconductor laser amplifier which emits first light from a first end surface thereof, receives second light from the first surface, and emits third light from a second surface thereof; a wavelength selection unit which receives the first light, selects a wavelength from among a plurality of wavelengths included in the first light according to strength of an electric field applied to the wavelength selection unit, and returns to the tapered-stripe semiconductor laser amplifier a portion of the first light having the selected wavelength as the second light; and an electric field applying unit which applies the electric field to the wavelength selection unit.
According to the present invention, the selected wavelength can be changed by varying the strength of the electric field applied to the wavelength selection unit, and therefore the selected wavelength can be precisely tuned by appropriately adjusting the strength of the electric field applied to the wavelength selection unit. In addition, since the variable-wavelength semiconductor laser device according to the present invention contains no mechanical driving means for changing the selected wavelength, the size of the device can be reduced compared with the conventional device which uses the mechanical driving means, and a stable output can be obtained.
(i) In the above variable-wavelength semiconductor laser device according to present invention, the wavelength selection unit may include a fiber grating which contains a core and a plurality of refractive index variation portions formed in the core at regular intervals, and reflects and diffracts the first light to determine the selected wavelength and return the second light to the tapered-stripe semiconductor laser amplifier. In this case, the electric field applying unit may include a pair of electrodes which are formed so that the plurality of refractive index variation portions are located between the pair of electrodes. In addition, the electric field applying unit may include a unit for applying the electric field between the pair of electrodes.
In the variable-wavelength semiconductor laser device described in the above paragraph (i), when an electric field is applied to the above plurality of refractive index variation portions in the fiber grating, the plurality of refractive index variation portions generate heat to change the volume of the plurality of refractive index variation portions. At this time, refractive indexes of the plurality of refractive index variation portions also change due to a thermooptic (TO) effect, and thus an effective grating constant (i.e., an effective pitch of the fiber grating) changes. Therefore, the effective grating constant, the selected wavelength, and the oscillation wavelength can be arbitrarily changed by changing the strength of the electric field applied to the plurality of refractive index variation portions in the fiber grating.
(ii) In the above variable-wavelength semiconductor laser device according to present invention, the wavelength selection unit may include a birefringent filter containing at least one birefringent element made of a material exhibiting an electrooptic (EO) effect, and a mirror which reflects the first light after the first light has passed through the birefringent filter. In this case, the electric field applying unit may include a pair of electrodes which are formed so that portions of the birefringent filter through which the first light passes are located between the pair of electrodes, and a unit for applying the electric field between the pair of electrodes.
In the variable-wavelength semiconductor laser device described in the above paragraph (ii), when an electric field is applied to the at least one birefringent element made of the material exhibiting the electrooptic (EO) effect, the refractive index of the at least one birefringent element changes due to the electrooptic (EO) effect. Therefore, the refractive index of the at least one birefringent element, the selected wavelength, and the oscillation wavelength can be arbitrarily changed by varying the strength of the electric field applied to the at least one birefringent element.
(iii) In the variable-wavelength semiconductor laser device described in the above paragraph (ii), the material of which the at least one birefringent element is made may be LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61), or doped with at least one of MgO and ZnO. When the birefringent element has a c axis, the pair of electrodes may be separated from each other in the direction of the c axis of the birefringent element.
(iv) In the above variable-wavelength semiconductor laser device according to present invention, the wavelength selection unit may include an optical waveguide element which contains a substrate showing an electrooptic effect, an optical waveguide being formed in the substrate and having therein a reflection grating which reflects and diffracts the first light to determine the selected wavelength and return the second light to the tapered-stripe semiconductor laser amplifier. In this case, the electric field applying unit may include a pair of electrodes which are formed so that the reflection grating is located between the pair of electrodes, and a unit for applying the electric field between the pair of electrodes.
In the variable-wavelength semiconductor laser device described in the above paragraph (iv), when an electric field is applied to the above reflection grating formed in the optical waveguide, an effective grating constant (i.e., an effective pitch of the reflection grating) changes due to the electrooptic (EO) effect. Therefore, the effective grating constant, the selected wavelength, and the oscillation wavelength can be arbitrarily changed by changing the strength of the electric field applied to the reflection grating in the optical waveguide.
(v) In the variable-wavelength semiconductor laser device described in the above paragraph (iv), the substrate may be LiNbxTa1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61), or doped with at least one of MgO and ZnO.